If you think about it, even if Official Guide questions were evenly distributed over the difficulty curve, half of them would be relatively easy, and others would still not be particularlly hard. So, most OG Quant questions aren't very hard.
Meanwhile, I'm pretty sure only a few OG Quant questions are close to the hardest questions on the actual GMAT in difficulty. So, the difficulty of OG questions is not really represenatative of the difficulty of the questions you'll see when taking the test if you're scoring relatively high.
When you take the GMAT, the questions will be similiar in difficulty to the questions you see on the official practice tests. So, your prep should be aimed and your developing skill in getting such questions correct. Accordingly, you should work on increasing you accuracy for 700-level questions.